The present invention relates to a display device with an active matrix of electrowetting cells (EW cells), where the liquids in the EW cells can be brought to a defined initial condition quickly between two value writing operations, so that fast switching times can be achieved.
The field of application of the invention includes display devices which comprise as an active matrix a controllable spatial light modulator with light modulation elements which are realised in the form of electrowetting cells. These display devices preferably serve to display three-dimensional information, in particular video sequences, autostereoscopically or holographically.
Because the EW cells serving as light modulation elements have great advantages over other types of light modulation elements, as they have e.g. greater brightness and contrast, large viewing angles and a great switching speed, they enjoy good applicability in many fields. The actual physical form of these cells must be adapted to the specific application; this regards e.g. also the control mechanisms of the EW cells. EW cells with these properties can preferably serve as elements of a controllable active matrix and be used in a display device. The design of EW cells is generally known to persons skilled in the art and shall thus only be mentioned briefly here. A simple EW cell comprises an enclosed container with a top and a bottom faceplate, which are preferably made of a transparent material, and between which two immiscible liquids form an interface. The inclination of that interface can be controlled with the help of a number of electrodes, where the control is effected by varying the voltage supplied to the electrodes. Depending on the actual electrode arrangement and control, the interface can realise the function of a prism and/or that of a lens.
When using controllable EW cells in display devices, a general problem is that—in contrast to the light modulation elements of controllable liquid crystal cells (pixels) of an LC display panel—a relative large volume of liquids must be moved when controlling the EW cells of the active matrix. In the EW cells of autostereoscopic display devices, greyscale values must be realised in a very short time in order to modulate incident light. In display panels of holographic display devices which work in real-time mode, different phase and/or amplitude values must be realised in a very short time in order to modulate incident light. In either case the electric charges representative of said values must be varied very quickly in the EW cells. In a normal LCD panel, the pixel capacitors can be subjected to a charge reversal very quickly because their capacitance is relatively small and constant. If in an LCD panel the pixel is activated for a short time by supplying a voltage to the row line, then the voltage of the pixel capacitor can be varied through the data line. If the pixel is deactivated again, the voltage in the capacitor, which is now insulated, will remain constant over the entire duration of the image representation (e.g. 16 ms) in an LCD panel, because the capacitance does not change. A new greyscale value will only be set after a certain switching time (about 2-20 ms) by way of supplying a different voltage.
In the display device with EW cells, there is also a switching time, within which the interfaces of the liquids in the EW cells adapt to a new setted control voltage. As the wetting in the EW cells changes, their capacitance will vary as well. Because the charge in the insulated electrode which is not intended to control an EW cell (cell capacitor) is constant, the voltage will change as well according to the equationQ=C*U  (1)
where Q is the charge, C is the capacitance, and U is the voltage.
Therefore, the desired inclination angle of the interface and thus the voltage value of an EW cell which corresponds with this desired inclination angle cannot be determined by writing a charge which is only controlled by the voltage to the cell electrodes once. It is always necessary to take into consideration at both electrodes the actual level of the liquids which is effected by a previous switching operation. This level of the liquids, however, is often not known or cannot be determined precisely, and it is often not identical at the two electrodes either. This is why the EW cell is brought to a defined initial condition e.g. by a resetting operation between two switching operations or two addressing operations, i.e. before a desired value is written. This can be done e.g. by readjusting the supplied voltage and/or by keeping it at a constant level as long as it takes for the interface to achieve a steady state. Since the readjustment in an EW cell takes a relatively long period of time because of the volumes of the liquids to be moved, it does not make sense any more to realise the resetting operation this way.
A resetting operation here means the process of restoring a defined initial inclination of the interface in the EW cell. This means that a voltage or capacitance which is present in the EW cell after a writing operation must be reset before a new writing operation with a new value can be effected. Otherwise the newly written value will be represented wrongly in the active matrix. A reconstruction of a 3D object which results from such wrong values will then exhibit reconstruction errors.
Document WO 2007/049196 describes one possibility of a resetting operation in a display device with an active matrix of EW cells. Each EW cell here has a connection to an additional reset line. If a voltage is supplied to the reset line which is greater than the forward voltage of the diode, the EW cell will be charged at that voltage, irrespective of the writing operation. It is thus possible to initialise the resetting operation in EW cells of a certain row, while values are being written to the EW cells in the other row. One disadvantage of this type of display is that it is only possible for the EW cell to be charged at the maximum or the minimum voltage to be supplied, depending on the polarity of the diode. Charging at the maximum voltage is particularly disadvantageous in an EW cell which is not of a flat design, but which has walls. At the maximum voltage, the EW cell has the lowest capacitance and can thus only accept a very little charge. A further disadvantage is that the charge for the resetting operation must be supplied from outside to the active matrix, which thus represents an additional power loss. This is particularly critical in appliances with display devices of the described active-matrix type, which require low switching delays.
The resetting operation described above is rather disadvantageous when importance is attached to fast switching, as is required for example in the display panel of a holographic display device. It is in particular the fact that only single rows are activated that takes too long in real-time applications. In contrast, charging the electrodes at a medium potential would be much more beneficial. The EW cell then exhibits about half its maximum capacitance, which is a better starting point for writing a value compared with the prior art. Twice the voltage to be achieved must now again be supplied for maximum wetting with water. Also the achievement of the minimum voltage value up to a small remaining capacitance can be controlled much better from a maximum switching state.